Subsurface flowmeter



Dec. 25, 1951 F. MORGAN ETAL 2,530,182

SUBSURFACE FLOWMETER Filed May 1, 1947 5 Sheets-Sheet 1 w 2 m 3m m? 1 EM m lill'flllam i Dec. 25, 1951 F. MORGAN EI'AL 2,580,182

SUBSURFACE FLOWMETER Filed May 1, 1947 3 Sheet s-Sheet 2 Fig.7.. Fig-5- INVENTORS Fr ank Mar an '1] EnzE1W-Ree|:[ Morris, Muskat Dec. 25, 1951 F. MORGAN ETAL SUBSURF'ACE FLOWMETER 5 Sheets-Sheet 3 Filed May 1, 1947 D.. Em-

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UNITED STATES PATENT OFFICE SUBSURFACE FLOWMETER Frank Morgan, Fox Chapel, Denzel W. Reed, Penn Township,

Allegheny County,

and Morris Muskat, Oakmont, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Application May 1, 1947, Serial No. 745,330

8 Claims. 1

This invention relates to apparatus for the measurement of fluid flow, particularly the measurement of fluid flow in wells.

It is often desirable for the operator of a gas or oil producing well to know the rate at which fluid is flowing from different levels or zones in the well bore. Similarly, in the event the well is used for pressure maintenance, it is important for the operator to know into which sand or sands the injected fluid is passing. Obviously knowledge of this kind will enable the operator to 'ncrease the efliciency of production of the hyirocarbon fluids.

An object of the invention is to provide apparatus for the measurement of the rate of fluid flow through various passages in a well bore, casing, and the like.

Another object of the invention is to provide apparatus for measuring the rate of fluid flow at various depths in a Well bore.

A further object of the invention is to provide an apparatus for locating the productive zones in a formation and to determine the rate of flow of fluid from such zones.

Still another object of the invention is to provide an apparatus for locating the zones of entry of fluids in injection wells and to determine the rate of fluid flow into said zones.

Another object of the invention is to provide a fluid flow measuring apparatus including a variable resistance element having a high negative temperature coefiicient of electrical resistance, which is adapted to be interposed in the path of the fluid, and including means for measuring the electrical resistance of the resistance t element.

A further ob ect of the invention is to provide a fluid flow measuring apparatus including a plurality of variable resistance elements having a high negative temperature coefficient of electrical resistance and adapted to be interposed in the path of the fluid, and including means for regulating the current flowing through said resistance elements, and for measuring their electrical resistance, whereby the rate and characteristics of fluid flow may be measured and recorded.

Still another object of the invention is to provide a fluid flow measuring apparatus employing an electrical resistance element commonly known as a thermistor and which is adapted for measurement of low ates of fluid flow with considerable accuracy, and without any substantial obstruction to the flow of the fluid itself.

Other objects and advantages of the invention will become apparent from the following description of a preferred embodiment thereof, as illustrated in the accompanying drawings, and in which,

Figure 1 is a longitudinal sectional view of one form of the invention, illustrating a flow measuring device adapted for being lowered into a well for determining the rate and characteristics of flow therein,

Figure 2 is an enlarged fragmentary sectional detail of the parts located generally at A of Figure 1,

Figure 3 is an enlarged fragmentary sectional detail of the parts located generally at B of Figure 1,

Figure 4 is a sectional plan view taken on line 4-4 of Figure 1,

Figure 5 is a face view of a thermistor element and mounting therefor, which may be employed in the devices of Figures 1 and 7,

Figure 6 is a schematic electric wiring diagram showing a current limiting and measuring circuit which may be employed in conjunction with the form of device shown in Figure 1,

Figure '7 is a longitudinal sectional view of a modified form of the invention, adapted for being lowered into a well and illustrating a type of meter using two or more flow measuring elements for differential measurement of fluid flow,

Figure 8 is a schematic electric wiring diagram showing a form of the current limiting and measuring circuit used with the differential meter of Figure 7,

Figure 9 is an elevation, partly in section, of an alternative type of top or bottom which may be used on the apparatus of Figs. 1 or 7,

Figure 10 is a top view accompanying the elevation of Fig. 9, and

Figure 11 shows typical calibration curves for the flow meter of our invention.

In order to understand clearly the nature of the invention, and the best means for carrying it out, reference may now be had to the drawings, in which like numerals denote similar parts throughout the several views.

Referring to Fig. 1, there is a flow measuring element l0, and a temperature measuring element l2, separated by such a distance that radiation from element ID has practically no eflect on element l2. These elements are mounted in insulated rings l4 and 5 respectively, as shown in Fig. 1.

The elements l0 and I2, consist preferably of a semi-conductor of the type known to the trade as a varistor or "thermistor having the gen- 3 eral appearance of a small capsule and disposed in the opening 39 (see liigure 5) in the insulated rings I 4 and I6 respectively, as shown in Fig. 1, and in detail in Fig. 5. The thermistor It! has two lead wires H and 13 which are connected to the conductive bushings l5 and I1 seated in apertures formed in the rings I 4 and I6. It will be seen that a small conductive spring 9 may be interposed in the lead wire l3 to provide a certain amount of resiliency in the thermistor mounting. The bushings such as l5 and I1, may extend all the way through the rings l4 and I6 so as to permit contact to be made therethrough, or may extend only to one face surface of the rings, depending upon the arrangement of contacts to be made. Additional metal bushings, such as the one shown at 2| (Fig; 5) may also be provided in the rings. The bushing 2| is used to provide conductive contact through the upper "thermstor ring l6 for the electrical connection runnin to the lower thermistor" ring M as will become evident later. A small hole I in the rings is provided to engage a pin (not shown) for maintaining proper orientation and to prevent rotation of the ring during assembly in the apparatus. The thermistor element I is tautly suspended by its lead wires H and i3 and spring 9 between small transverse holes in bushings i and H, the outer end of the lead wire being bent over and soldered to the respective bushing as illustrated clearly in Fig. 3, the insulating ring l4 having radial outer openings to permit access to the bushing for soldering the lead wires to the bushings.

The unique property of the thermistor which renders it particularly valuab e as a flowmeter element, is its high negative temperature coefficient of resistance. Such "thermistors are well known in the art, several types having be n described in various patents, such at 2,373.160; 2.396.196; 2,339,029; and 2,332,596. It will be understood that citation of the above patents is merely by way of reference and is not to be taken as a limitation of the term thermistor" when used herein, to only those forms described in the patents.

As an example of the properties of thermistors," at a temperature of '77 degrees Fahrenheit, the resistance of one form of "thermistor will decrease by approximately 2.4 per cent of its original value for an increase of temperature of one degree Fahrenheit, while platinum will increase in resistance by less than 0.2 per cent for the same temperature change. Since the thermal type of flowmeter depends upon loss of heat in an element due to gas or fluid flow, the thermistor thus possesses an advantage over the older metallic type of element. "Thermistors possess another advantage in this particular application due to their higher specific resistance. At room temperature the resistance of the thermistor" element mentioned above is approximately 40,000 ohms; at 212 degrees Fahrenheit this resistance decreases to 3,500 ohms, and then to 950 ohms at 300 degrees Fahrenheit. In comparison to these values the resistance of 2,000 feet of cable is entirely negligible, and the problem of recording is thus greatly simplified. All the control apparatus including vacuum tubes, batteries, and other parts, may remain at the surface of the ground while the thermistor itself is run in the hole.

Semi-conductors, of which the thermistor is an example, have unique resistance characteristics which are made use of in our invention.

Ordinary conductors such as metals have an electrical resistance which varies with temperature in a substantially linear fashion, represented by the well-known relation where R0 is the resistance at the temperature to, Rt is the resistance at a temperature t and a is the well-known temperature coefiicient of resistance which depends on the material. Semiconductors however have a resistance which varies with temperature in a non-linear fashion which may be represented by the relation,

where R0 is the resistance at the temperature To (Kelvin), R is the resistance corresponding to the temperature T, e is the base of natural logarithms and B is a constant which depends on the semi-conducting material. This relation is known to apply to the semi-conductors such as "thermistors and in one type of thermistor used in our invention the constant B is approximately 4000. It is seen that such semi-conductors are characterized in having a resistance which varies in a highly non-linear manner and also in having a substantial decrease in resistance as the temperature rises.

The above resistance vs. temperature characteristic has been combined in our invention with the cooling effect of the flowing fluid in such a way as to produce a relation of voltage vs. flow which has large slope at low flow rates resulting in high instrument sensitivity at low flow rates. Figure 11 shows typical calibration curves of our invention illustrating this result. The manner of obtaining these curves and their use will be described more fully later, but from Figure 11 it is apparent that the curves rise very rapidly at low values of flow and taper oiT at high flow rates. This is a particularly desirable feature in surveying wells because the high sensitiv ty at low flow rates makes possible the detection and measurement of small flows through leaks in casing, porous stringers in open formation, etc. whereas the lower sensitivity in the region of high flow rates results only in reduced ability to detect small differences which are unimportant at high flow rates. Additionally, the change in sensitivity is such as to compress the voltage scale at the high flow rates, thereby providing an extended useful range of the device.

In general, at very low values of current, Ohms law is obeyed by thermistors. At higher current values the voltage across the thermistor reaches a maximum and then decreases. Beyond this maximum point the thermistor acts as a negative resistance, that is, as the current increases the voltage drop across the element decreases. At still higher values of the current, usually beyond the safe working range, the slope of the voltage-current curves may again become positive.

While the property of negative resistance is of great value in many applications, it is apparent that the interpretation of results is often complicated or impossible when several values of current may correspond to a single voltage. Furthermore, trouble may be encountered in properly selecting a voltage such that a current range capable of measurement will obtain for the large flow variations that may be found in a given well.

In order to avoid these undesirable qualities of a constant voltage, the flowmeter here described has been designed to operate at essen- Q2) daily a constant current, pentode type vacuum tubes with cathode bias being used as current stabilizers.

Referring again to Fig. 1, it will be seen that the element rings I4 and I 6 are mounted in a suitable protecting tube I8, which is open in a longitudinal direction as much as possible, so as to offer a minimum of impedance to fluid flow therethrough along its axis.

The tube I8 is formed of several lengths or sections I9, threadedly joined together as at 2|] (see Fig. 3 for details), the joints being sealed by means of packing rings 22 or other suitable material. The tube sections I9 may be recessed at their ends as at 24 to receive annular bushings such as those shown at 26 and 28 in Fig. 3, for supporting the insulating rings I4 therebetween, a tight seal being effected in any suitable manner as by packing rings 30 to prevent fluid from reaching electrical connections. The packing rings 30 have holes opposite the bushings I5, I'I, 2|, so that contact may be made to these, and rings ga ing a pin to prevent turning in assembly.

The bushings 26 and 28 are also lodged in annular recesses 32 formed in the ends of inner tube s ctions 34 which are spaced from the outer tube I8 so as to form an annular passageway 36 therebetween. The axially aligned inner bores 38 of the inner tube sections and the intervening axial bores 39 of the rings I4 thus form a tubular passageway 46 extending axially through the length of the main body portion 42 of the device shown in Fig. 1, the lower end 44 of which forms a discharge opening.

The bushings 26 and 28 carry spring contactors, examples of which are shown in Fig. 3. These contactors are of conventional type. and may be enclosed in an insulating sleeve as shown at 21 in Fig. 3. Certain of these spring connectors serve to make connections to ground. 1. e. to the case of the apparatus, in which case the insulating sleeve 21 is omitted and the metal shell of the contactor thereby grounds to the annular bushing 26 and the case section I9. The spring contactors are connected in circuit by wires running in the annular space 36. Similar spring contactors are held in annular bushings just above the ring I4 and just below the ring 52 described later.

An upper extension 46 having an axial bore in alignment with bore 40, is secured to the upper end 48 of the main body portion 42, by means of a coupling 50 threaded thereto. The extension 46 is reduced at its upper end 54, and has external threads 69, whereby it may be engaged ,with pipe stem sections, or a suspension cable clamp, not shown, for being supported. The cable 68 may extend upwards through a stufling box or so called lubricator at the well head.

Two insulating rings 52 and 53 being seated in the upper end of the body 42 and retained therein by the coupling 56 as shown in detail in Fig. 2. The rings 52 and 53 are similar to the insulating rings I4 and I6, but do not have any flow measuring elements such as those shown at III and I2 supported therein. The upper ring 53 is a plain insulating ring without metal bushings. The lower ring 52 has metal bushings located similarly to I5, I1 and 2| of the rings I4 and I6.

The three conductor reinforced electric cable 68 may serve both to support the apparatus when lowered into a well and also to make electrical connection to the thermistor elements I!) and 36 also have a hole opposite 1 for en- I2 in the apparatus. The cable 68 enters through a conventional cable clamp (not shown) )which engages threads 69 at the top of the apparatus, then engages a waterproof separable connector 61. The lower end of the cable .68 is stripped of its outer Jacket and the three individual conductors each retaining its own insulation, are fanned out and each conductor soldered to its respective one of the three'bushings in the ring 52. One of the conductors will be grounded as explained later and may comprise the cable reinforcing.

Connections are made from the cable 68 to the thermistor elements IO'and I2 as follows. Referring to Fig. 1, one of the conductors from cable 66 is contacted by spring connector 49 held in an annular ring similar to rings 26 and 28 referred to in Fig. 3. The connector 49 is connected by wire 5| to a similar spring connector in the ring 26. Contact is thus made to bushing I I of ring I6 and to the thermistor I2 mounted therein. The bushing I5 of ring I6 is contacted by a diametrically opposite spring connector (not shown) which is grounded to the case of the apparatus.

A second conductor from cable 68 goes to a bushing in ring 52 and is contacted by spring contactor 51 which is grounded to the case of the apparatus. Thus a completed circuit is established through "thermistor I2 two conductors of cable 68, one of which is grounded to the case of the apparatus.

The third conductor from cable 68 goes to the the connection goes via wire 6'! to spring connector 89 which contacts the bushing I5 in ring I4, and thus connects with the thermistor I0 mounted therein. The bushing IT is connected to ground by grounded spring connector II, thus completing the circuit through thermistor III.

The flow elements I6 and I2 through contact with their conductive bushings I5 and II, are thus connected electrically through the body 42 of the meter and cable 68 which goes upwards through the well to the surface of the ground, and is connected there to a current limiting and measuring circuit of suitable design, one form of which is shown in Figure 6. As seen in that figure, the flow measuring element I0 is thus connected across terminals I6 and I2, and the temperature measuring element I2 is connected across the terminals I0 and I6, the common connection in being the grounded conductor of the cable.

The circuit of Fig. 6 is so arranged that two essentially constant currents are supplied to elements I6 and I2. Element I2 has a small enough current passed through it so that there is substantially no heating effect of the element. Consequently, fluid flowing by this element causes a resistance change only because of an ambient temperature change.

The flow element ID has a much higher current passed through it, and consequently is heated to much higher than ambient temperature. Fluid flowing in the meter. causes a cooling of this element, andthus its resistance and the voltage across it'increase for a constant current.

The currents through the elements In and I2 are shown in milliammeters I8 and 8|! respectively, while the voltages across the elements are exhibited by voltmeters 82 and 84. As will be understood by those skilled in the art, the circuit illustrated in Fig. 6 includes a pair of multi-element electron tubes 88 and 88 having indirectly heated cathodes, and having plate voltage connected at 88, the resultant plate currents for each tube flowing through the flow and temperature measuring elements I8 and I2 which are interposed in their respective plate circuits.

As an example, which is illustrative only and not to be taken in a limiting sense, the plate voltage supply 98 may be on the order of 250 volts, and the fixed resistor 92 on the order of 200,000. ohms, the variable rheostats 94 and 96 having maximum values on the order of 1000 and 70,000 ohms respectively.

The voltage changes of both elements I8 and I2 are thus shown on the measuring part of the currentlimiting circuit, as well as on a recording voltmeter which may be connected in the circuit.

When the apparatus is used in the manner described various rates of flow past the element I8 of the apparatus of Figure 1 give rise to ccrresponding readings of voltmeter 82 of Figure 6. Calibration data may be obtained by reading the voltmeter 82 at various flow rates taken at constant temperature throughout the range of flow to be encountered and from such data a curve relating the indication of meter 82 to the actual flow may then be drawn. Since the cooling effect of the flow on the element I8 will depend on the temperature of the flowing fluid, a different curve is obtained for diflerent temperatures and a family of such calibration curves may be plotted as illustrated in Figure 11. Such calibration tests may be made in the laboratory by blowing known quantities of gas of known temperature through the apparatus and observing the indications of meter 82. Alternatively, a satisfactory and simpler calibration may be made with the apparatus in the well just prior to its use in a manner to be described later. The result of such calibration data is that thereafter one may infer the flow passing through the apparatus of Figure l in the well by observing the indication of meters 82 and 84 of Figure 6 at the surface of the ground. Thus the reading of the temperature-indicating meter 84 (previously calibrated as explained below) determines which one of the curves of Figure 11 applies, and from the reading of the flowindicating meter 82 as entered on this particular curve one may determine the flow through the apparatus directly in cubic feet per minute.

Similarly the element I2 may be calibrated for various temperatures by passing gas of known temperatures through the apparatus of Figure l and observing the indication of meter 84 of Figure 6. Since there is little or no heating of element I2, the indication of meter 84 is substantially independent of flow rate. From such calibration data a curve (not shown) may be drawn relating the temperature of the gas flowing through the apparatus to the indication of meter 84 and subsequently from this curve the temperature of flow encountered in the well may be inferred from the indication of meter 84 at the surface of the ground.

Calibration of. the form of the apparatus illustrated in Figures '7 and 8 is similarly carried out with certain modifications to be described later.

In one form of the apparatus shown in Fig. 1, fluid flowing into the well bore is directed into the bore 48 through slots 55 formed in the wall of extension 48 (see also Fig. 4), which may be covered by wire mesh 41 to block entry of debris and solid matter. Flow of fluid through the apparatus is accomplished by the use of a rubber packer ring 58 (Fig. 1) the outer diameter of which is slightly larger than the casing or open hole forming the well bore, in which the apparatus is being used. The fluid enters through the slots 56, passes along bore 48 through the entire length of the apparatus, and out through discharge opening 44 into the well bore, the direction of flow being indicated generally, for example, by the arrow 68. It will be seen that the packer ring 58 may be secured in position between two annular bushings 62 and 64, one of which is welded to the tube I8, and which are secured together with the packer ring therebetween, by means of screws 66.

In another form of the invention shown in Fig. 1, the packer ring 58 may be eliminated from the body 42 of the apparatus. Less fluid then goes through the apparatus bore 48, and the sensitivity is decreased. To use the apparatus in this form, it is desirable to take caliper logs of the well being measured, so that corrections can be made for the variations in sensitivity caused by variations in hole diameter.

In using the apparatus of Fig. 1 without the packer ring 58, the top of the apparatus should be left open as far as possible to permit unrestricted axial flow of fluid through the bore 48. To facilitate the entrance of fluid into the apparatus a funnel attachment such as is shown in Figs. 9 and 10 may be used, although this is not necessary. A funnel attachment similar to that of Figs. 9 and 10 may also be used on the bottom end of the apparatus should such be found desirable.

In using the attachment shown in Figs. 9 and 10 the parts 66, 64, 58, 62, 50, 46 of Fig. 1 are omitted or removed and the part 58 replaced by an internally threaded section I68 having a fun nel shaped upper portion I8I. The part I68 may be screwed on to the upper portion 48 of the apparatus of Fig. 1. To the inside of the flaring portion I6I there are welded four narrow angle pieces I 82 their upper ends approaching relative- 1y close to each other, and each angle piece having at its upper end an eye I63. The four eyes are attached to a conical head piece I64 having a central hole for the cable 68 and having in its base four narrow slots I65 into which the four eyes I63 of the four angle pieces fit. Through the conical head piece I84 and each eye I63 there is a pin I66 each of which is held in place with a cotter pin I81. A separable waterproof cable connector I68 is attached to one of the angle pieces I52 by means of a bracket. A cable clam I88 may be incorporated as part of the conical head piece I64, or the cable may have a clamp below the piece I84, said clamp (not shown) taking thetension strain against the bottom face of-"I8.4.'--- The open space between the narrow angle pieces I62 may be covered with screening I78 held in place by strips III and screws I'I2 into the angles I82 as indicated. The arrangement of Figs. 9 and 10 permits relatively unrestricted flow of fluid through the apparatus and at the same time serves to keep out dirt or solid matter.

Figure '7 shows a form of the apparatus which may be used for differential flow measurements. As shown, 98 and I88 are the flow measuring elements mounted in insulating rings I 82 and I84, the elements and insulating rings being similar to those shown in Figs. 1 and 5 and already described. Both the 'elements 98 and I88 are used to measure flow in this form of the apparatus, and are separated by the distance apart of the required difierential measurement.

The elements 08 and I are mounted in a hollow tubular housing generally indicated at I06, formed of a number of intercoupled tubular sections such as those shown at I08, H0, H2 and H4, the housing I06 having an axial bore II6 extending therethrough with a discharge outlet H8 at its lower'end. The upper end section I2.) is reduced at its upper end I22 which is threaded externally to permit coupling to pipe stem sections (not shown) if desired or to acable clamp for supporting the apparatus for raising and lowering it in a well bore.

Fluid flow in the bore hole may be diverted through the apparatus by use of the packing ring I24 which is slightly greater in diameter than the well bore, and is held in position between annular bushings I26 and I28 which are secured together by screws I30. The fluid enters through openings I32, flowing, for example, in the general direction of the arrow I34, through bore I I6.

Fluid flow across element I60 causes a resistance change in that element, and consequently a change in the voltage drop across it, in the same manner as explained in connection with Fig. l, but, if the apparatus of Fig. 7 is in a permeable section of the formation the fluid can escape through slots I36 formed in the housing I06 and into the sand. The remaining fluid continues through the bore IIB across element 98 and then out through discharge outlet I I8.

The fluid that has escaped through slots I36 must go into the formation because the path outside the apparatus is restricted by a second packer ring I38 similar in size and construction to packer ring I24 but located beyond the slots I36, and held in position between bushings I40 and I42. The remaining fluid that continues through the apparatus and past element 98 has less velocity and therefore the effect on element 98 is less than the full flow on element I00.

The elements 98 and I00 are connected electrically through the body of the apparatus housing I06 and cable I44 to the surface, and into a suitable circuit such as is shown in Fig. 8. Electrical connection inside the apparatus housing I06 and leading to the thermistor elements 98 and I00 are indicated diagrammatically in Fig. 7 and the details of such connections inside the apparatus may be similar to those described in connection with Fig.' 1 already described. The leads 99, ISI and I53 (Fig. 8) are the conductors in the cable I44, lead 99 being the grounded conductor.

The circuit of Fig. 8 is so arranged that the thermistor elements 98 and I00 are inserted in the plate circuits of multiple element thermionic tubes I46 and I48, which are supplied with high voltage from a source connected at I49. The plate currents in both tubes are held constant and preferably equal to each other, the currents being shown on milliammeters I50 and I52. The voltage difierential, if any, is read on voltmeter I54 bridged across the two tube plates, and may be recorded on a continuous recorder, not shown, if desired.

The differential form of the apparatus shown in Fig. 7 may also be used without the packers I24 and l38. If desired on omission of the packers the apparatus may be equipped with top and bottom funnel shaped outlets similar to that shown in Figs. 9 and 10.

The following instructionsmay serve to outline the calibration procedure when such is to be carried out in a well, for example, a gas-Injection well, instead of in the laboratory. An advantage results when the calibration is carried out in the particular well in which the apparatus is to be used in that the temperature has been found to remain substantially constant at the calibration value, thus eliminating the need for observing the temperature-indicating meter 84 of Figure 6, and in fact when the apparatus is to be so calibrated the temperature element I2 of Figure 1 and its connected electrical equipment on the right-hand side of Figure 6 may be dispensed with entirely thus further simplifying the construction of the apparatus. Also when such a calibration is made in the particular well to be tested the calibration curve (see Figure 11) becomes but one curve instead of a family of curves thus simplifying the computation of the actual flow encountered.

The general procedure used in calibrating the flow meter is to lower it into the well to a point 5 or 10 feet above the casing seat and vary the flow into the well. For each calibration point the flow is held essentially constant until equilibrium has been established. .This procedure is suitable for the apparatus of Fig. 1 because the variation in flow is measured across one thermistor, namely by means of the meter shown as 82 in Fig. 6. When the differential form of apparatus, Fig. 7, is used in the casing as above, the flow across both thermistors is the same because no fluid can escape through slots I36 (assuming diaphragm packers are used). Since the voltage measured in the differential form of apparatus as shown in Fig. 8 is the difference in the voltage drops across the thermistors, no voltage difierence will be observed on meter I54 for equal flows through the two thermistors. This will hold because the thermistors will have identical characteristics for the same conditions of flow and temperature. To obtain a calibration of the diflferential form of the apparatus, it is necessary to cut one thermistor out of the circuit. This is readily accomplished by removing either tube I46 or I 48, Fig. 8, and connecting voltmeter I54 across the appropriate thermistor, S8 or I00, through which current flows. The calibration is then carried out on one thermistor" in the same manner as for the apparatus of Fig. 1. Since the thermistors are selected so as to have identical characteristics, the above described calibration of one element is all that is necessary to calibrate the differential form of the apparatus. In the difierential form, one thermistor element may serve as a temperature indicator by employing the circuit of Fig. 6. This is useful to determine the temperature at which a calibration is made.

If the diaphragm packers are omitted from the apparatus the same calibration procedure may be used. However in this case the apparatus tends to become a velocity meter, and its absolute flow calibration depends on the diameter of the hole or pipe in which it is suspended. Small local variations in hole diameter have little effect on the calibration. Large variations due to caving of formation wall, etc. will affect the calibration when no packers are used, and will also'affeet the reading if packers are used which are too small in diameter to completely bridge the enlarged hole.

It will be seen that this invention describes apparatus which has extreme sensitivity for measuring fluid flow. This device can easily measure fluid flow on the order of as little as I 500 cubic feet of gasper day. Hot wire anemometers as heretofore constructed, generally have low sensitivity for high flow rates. However, the present invention permits operation at constant current and at variable voltage drop across the thermistor elements. A convenient temperature at which the thermistor may be operated is about 160 degrees centigrade above ambient. In actual use, no fluid flows have yet been encountered the magnitude of which is great enough to produce a low order of sensitivity.

Although we have described preferred forms of our invention in speciflc terms, it is to be understood that various changes may be made in size, shape, materials and arrangement without departing from the scope and spirit of the invention as claimed herein.

The calibrating procedure described herein is by way of example only and our apparatus may also be calibrated by passing known quantities of fluid through it at the surface of the well or in the laboratory.

We claim:

1. A flow meter comprising a housing adapted for being lowered into a well and having a, passageway extending longitudinally therethrough, means carried by said housing to induce well fluid to flow through said passageway, semi-conductor means disposed in the said passageway in the path of said fluid and having the characteristic that its electrical resistance decreases in a nonlinear manner with increase in operating temperature, means electrically connected to said semi-conductor means for maintaining a constant electric current flowing through said semiconductor means, and indicating means electrically connected to said semi-conductor means responsive to variations in electrical resistance thereof, whereby the characteristics of said fluid flow may be determined.

2. A flow meter comprising a housing adapted for being lowered into a well and having a passageway extending longitudinally therethrough, means carried by said housing restricting passage of fluid outside said housing, such fluid being free to flow through said passageway, semi-conductor means disposed in the path of said fluid and having the characteristic that its electrical resistance decreases in a non-linear manner with increase in operating temperature, an electron discharge tube, said semi-conductor means being connected to the anode of said tube, a circuit including said tube and semi-conductor means and so arranged as to maintain the electric current flowing through said semi-conductor means at a constant value, and indicating means electrically connected to said semi-conductor means responsive to variations in electrical resistance thereof.

3. A flow meter comprising a housing adapted for being lowered into a well and having a passageway extending longitudinally therethrough, a packer ring carried by said housing and extending outward therefrom to block passage of fluid between the housing and the well wall, such l2 electrically connected to said semi-conductor means responsive to variations in electrical resistance thereof.

4. A flow meter comprising an elongated hollow tubular housing closed at its upper end and having an open lower end, said housing having fluid inlet means formed adjacent its upper end through which fluid is free to enter the housing and flow therethrough to be discharged through its open lower end, a first thermistor? element supported in said housing in the path of said fluid, a second thermistor" element supported in said housing in the path of said fluid and spaced from said flrst thermistor" element, said thermistor elements being so constructed and arranged as to provide a minimum of obstruction to the fluid flow therepast, an electronic circuit in which said thermistor elements are connected and including electron discharge means for providing stabilized currents flowing through said thermistors, and indicating means electrically connected to said thermistors" responsive to variations in voltage drop across said thermistors.

5. A flow meter comprising an elongated hollow tubular housing closed at its upper end and having an open lower end, packing means carried by said housing intermediate its ends and extending outward therefrom to block passage of fluid between the housing and the well wall, said housing having fluid inlet means formed adjacent its upper end through' which fluid is free to enter the housing and flow therethrough to be discharged through its open lower end, a first thermistor element supported in said housing in the path of said fluid, a second thermistor element supported in said housing in the path of said fluid and spaced from said first thermistor" element, said thermistor elements being so constructed and arranged as to provide a minimum of obstruction to the fluid flow therepast, an electronic circuit in which said "thermistor elements are connected and including electron discharge means for providing stabilized currents flowing through said thermistors, and

indicating means electrically connected to said 50 said first and second flow measuring elements,

fluid being free to flow through said passageway,

-means being connected to the anode of said tube,

a circuit including said tube and semi-conductor means and so arranged as to maintain the electric current flowing through said semi-conductor means at a constant value, and indicating means means electrically connected to said thermistor elements for-causing constant current to flow through each of said thermistor elements, and indicating means electrically connected to said thermistor" elements responsive to the voltage drop differential between said thermistor elements, whereby characteristics of fluid flow in said well can be determined.

7. A differential flow meter comprising a housing adapted for being lowered into a well, and having a passageway extending therethrough, first packing means carried by said housing near its upper end and extending outwardly from the housing to block passage of fluid between the housing and the well wall, said fluid being free to flow through said housing passageway, a first fiow measuring thermistor element supported in said passageway in the path of said fluid flow, a second flow measuring thermistor element supported in said passageway nearer its discharge end and spaced from said first element, said housing having an intermediate opening formed in its walls between said first and second flow measuring elements, second packing means carried by said housing between said intermediate opening and lower discharge opening of said passageway to block the flow of fiuid between the adjacent portion of the housing and the well wall, means electrically connected to said thermistor elements for causing constant current to flow through each of said thermistor elements, and indicating means electrically connected to said "thermistor elements responsive to the voltage drop differential between said thermistor elements, whereby characteristics of fluid flow in said well can be determined.

8. A difierential fiow meter comprising a housing adapted for being lowered into a well, and having a passageway extending therethrough, well fluid being free to fiow through said housing passageway, a first thermistor element supported in said passageway in the path of said luid fiow, a second "thermistor" element supported in said passageway nearer its discharge end and spaced from said first element, said housing having an intermediate opening formed in its walls between said first and second thermistor elements, a pair of electron discharge tubes whose anodes are respectively connected to said thermistor elements, an electric circuit including said tubes and said thermistor" elements and adapted to maintain the electric current flowing through each of said thermistor elements at a constant value, and voltage-indicating means electrically interconnecting said thermistor elements responsive to variations in the difierence between between the voltage drops across said thermistor elements.

FRANK MORGAN.

DENZEL W. REED.

MORRIS MUSKAT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 853,492 Beck May 14, 1907 1,156,630 Savage Oct. 12, 1915 1,265,775 Hadaway May 14, 1918 1,440,778 Foster Jan. 2, 1923 1,652,472 Erwin et a1 Dec. 13, 1927 2,175,890 Glowatzki Oct. 10, 1939 2,352,056 Wilson June 20, 1944 2,375,273 Black May 8, 1945 2,379,138 Fitting, Jr., et al. June 26, 1945 2,421,759 Pearson June 10, 1947 

